1. Field of the Invention
The present invention relates to exposure apparatus, and device manufacturing methods, and more particularly to an exposure apparatus used in a lithography process where electronic devices such as semiconductor devices, liquid crystal display devices, or the like is manufactured, and a device manufacturing method that uses the exposure apparatus.
2. Description of the Related Art
In a lithography process for producing electronic devices such as semiconductor devices (integrated circuits), liquid crystal display devices, or the like, projection exposure apparatus are used that transfer the image of a pattern formed on a mask or a reticle (hereinafter generally referred to as a ‘reticle’) via a projection optical system onto each of the shot areas of a photosensitive substrate (hereinafter referred to as a ‘substrate’ or a ‘wafer’) such as a wafer, a glass plate, or the like whose surface is coated with a resist (photosensitive agent). As this type of projection exposure apparatus, conventionally, a reduction projection exposure apparatus by a step-and-repeat method (the so-called stepper) has been frequently used. However, recently, a projection exposure apparatus by a step-and-scan method (the so-called scanning stepper) that performs exposure by synchronously scanning the reticle and the wafer is also gathering attention.
The resolution of the projection optical system equipped in the exposure apparatus becomes higher when the wavelength (hereinafter also referred to as ‘exposure wavelength’) of the exposure light used becomes shorter, or when the numerical aperture (NA) of the projection optical system becomes larger. Therefore, the exposure wavelength used in the projection exposure apparatus is becoming shorter each year due to finer integrated circuits, along with the increase in the numerical aperture of the projection optical system. The exposure wavelength currently mainly used is 248 nm of the KrF excimer laser, however, a shorter wavelength of 193 nm of the ArF excimer laser has also been put to practical use.
In addition, along with resolution, depth of focus (DOF) is also important when exposure is performed. Resolution R and depth of focus δ can be expressed as in the equations below.R=k1·λ/NA  (1)δ=k2·λ/NA2  (2)
In this case, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. From equations (1) and (2), it can be seen that when exposure wavelength λ is shortened and numerical aperture NA is enlarged (increased NA) to increase resolution, depth of focus δ becomes narrow. In a projection exposure apparatus, when exposure is performed, an auto-focus method is used to make the surface of the wafer match the image plane of the projection optical system. Accordingly, it is desirable for depth of focus δ to have a certain amount of width. Therefore, methods have been proposed in the past to substantially widen the depth of focus, such as the phase shift reticle method, the modified illumination method, the multi-layer resist method, and the like.
As is described above, in the conventional projection exposure apparatus, the depth of focus is becoming narrow due to the shorter exposure wavelength and the increased numerical aperture. And, in order to cope with higher integration, the exposure wavelength is presumed to be shorter in the future. If such a situation continues, the depth of focus may become so small that margin shortage may occur during the exposure operation.
Therefore, as a method of substantially shortening the exposure wavelength while increasing (widening) the depth of focus when compared with the depth of focus in the air, an immersion exposure method (hereinafter also appropriately referred to as ‘immersion method’) has been proposed. In the immersion method, resolution is improved by filling the space between the end surface of the projection optical system and the wafer surface with liquid such as water or an organic solvent to make use of the fact that the wavelength of the exposure light in the liquid becomes 1/n of the wavelength in the air (n is the refractive index of the liquid which is normally around 1.2 to 1.6). In addition, in the immersion method, the depth of focus is substantially increased n times when compared with the case where the same resolution is obtained by a projection optical system (supposing that such a projection optical system can be made) that does not employ the immersion method. That is, the depth of focus is substantially increased n times than that in the air.
However, in the case the immersion method above is merely applied to a projection exposure apparatus by the step-and-repeat method, the liquid spills from the space between the projection optical system and the wafer when the wafer is moved in between shots by a step movement to the exposure position for the next shot area after exposure of a shot area has been completed. Therefore, the liquid has to be supplied again, and the recovery of the liquid could also be difficult. In addition, in the case when the immersion method is applied to a projection exposure apparatus by the step-and-scan method, because exposure is performed while moving the wafer, the liquid has to be filled in the space between the projection optical system and the wafer while the wafer is being moved.
Considering such points, a proposal has been recently made on ‘an invention related to a projection exposure method and a unit where a predetermined liquid flows along the moving direction of a substrate, so that the liquid fills in the space between the end portion of an optical element on the substrate side of a projection optical system and the surface of the substrate when the substrate is moved in a predetermined direction,’ (for example, refer to patent document 1 below).
Besides such a proposal, as a proposal for improving resolution as in the immersion exposure method, a lithography system is known that places a solid immersion lens in the section between a projection lithography lens system (projection optical system) and a sample (for example, refer to patent document 2 below).
According to the invention disclosed in patent document 1 below, exposure with high resolution and a larger depth of focus than the depth of focus in the air can be performed by the immersion method, and the liquid can also be filled in the space between projection optical system and the substrate in a stable manner, or in other words, can be held, even when the projection optical system and the wafer relatively moves.
However, in the invention disclosed in patent document 1 below, because the supply piping, the recovery piping, and the like are arranged outside the projection optical system, the degree of freedom is limited for peripherals such as sensors of various kinds like a focus sensor or an alignment sensor that have to be arranged around the projection optical system.
In addition, in the invention according to patent document 1 below, in the case there is a flow in the liquid filled in the space between the projection optical system and the substrate, when the liquid is irradiated by the exposure light on exposure, temperature inclination or pressure inclination relative to the direction of the flow may occur within the projection area of the pattern in the space between the projection optical system and the substrate. Especially when the space in between the projection optical system and the substrate is large, or in other words, the layer of liquid is thick, such temperature inclination or pressure inclination could be the cause of aberration such as inclination of image plane, which could lead to partial deterioration in the transfer accuracy of the pattern, which in turn could be the cause of deterioration in the line width uniformity of the transferred image of the pattern. Accordingly, the layer of liquid is preferably thin. However, in this case, the space in between the projection optical system and the substrate becomes narrow, which makes it difficult to arrange a focus sensor.
In addition, in the invention according to patent document 1 below, it is difficult to recover the liquid completely, and the probability was high for the liquid used for immersion to remain on the wafer after exposure. In such a case, temperature distribution in the atmosphere or a refractive index change in the atmosphere occurs by the heat of vaporization generated when the remaining liquid evaporates, and these phenomena could be the cause of measurement errors in a laser interferometer system that measures the position of the stage on which the wafer is mounted. Furthermore, the remaining liquid on the wafer could move to the back of the wafer, which could make the wafer stick to the carrier arm and difficult to separate.
Meanwhile, in the lithography system according to patent document 2 below, the distance between the solid immersion lens (hereinafter shortened appropriately as ‘SIL’) and the sample is maintained at around 50 nm or under. However, in the lithography system in the near future whose target is to transfer and form a fine pattern onto a sample (such as a wafer) at a line width of around 70 nm or under, when an air layer whose thickness is 50 nm exists between the SIL and the sample, it becomes difficult to obtain sufficient resolution in the image of the fine pattern referred to above. That is, in order to obtain sufficient resolution in the fine pattern above, the distance between the SIL and the sample has to be maintained at a maximum of 30 nm or under.
However, in the lithography system according to patent document 2 below, because a configuration using air bearings is employed to maintain the distance between the SIL and the sample, it is difficult to obtain sufficient vibration damping due to the nature of air bearings. As a result, the distance between the SIL and the sample could not be maintained at 30 nm or under.
As is described, in the conventional examples disclosed in patent documents 1 and 2 below and the like, various points are found that should be improved.
Patent Document 1: the pamphlet of International Publication Number WO99/49504
Patent Document 2: the description of U.S. Pat. No. 5,121,256